The Math That Protects Your Lungs

CADR isn’t complicated once you understand what you’re calculating. Here’s how to match a purifier to your space.

What CADR Actually Measures

CADR (Clean Air Delivery Rate) tells you the volume of completely clean air a purifier delivers per minute. It’s measured in CFM (cubic feet per minute) in the US or CMH (cubic meters per hour) elsewhere.
Three separate CADR ratings appear on most purifiers:

  • Smoke (0.09-1.0 microns): Use this number—it represents the hardest particles to capture
  • Dust (0.5-11.0 microns)
  • Pollen (0.5-3.0 microns)

The Simple Formula

Step 1: Calculate your room volume

  • Length × Width × Height = Room Volume (in cubic feet or meters)

Step 2: Multiply by air changes per hour

  • Room Volume × 3 (minimum) = Required CADR

Why multiply by 3? Your air should be completely filtered at least three times per hour for basic protection. For enhanced protection (like during flu season), use 5. The CDC recommends 5 air changes per hour for COVID-19 prevention.

Real Example

Room: 30 ft × 30 ft × 9 ft

  • Volume: 30 × 30 × 9 = 8,100 cubic feet
  • Required CADR: 8,100 × 3 = 24,300 cubic feet per hour
  • Convert to CFM: 24,300 ÷ 60 = 405 CFM minimum

Note: If CADR is listed in CMH (cubic meters per hour), divide by 1.7 to get CFM. If you see CFM, multiply by 1.7 to get CMH.

The AHAM 2/3 Rule

For rooms up to 8 feet tall, your purifier’s smoke CADR should equal at least 2/3 of your room’s square footage. A 150 sq ft room needs a minimum CADR of 100.

What the Tests Don’t Tell You

CADR is measured on the highest fan speed in a controlled 1,008 cubic foot chamber. In reality:

  • You might not run your purifier on high due to noise
  • Your room may have different air exchange with other spaces
  • Filter efficiency drops as filters age

Pro tip: Get a purifier with CADR slightly higher than your calculation requires. This gives you flexibility to run it on quieter, lower settings while still getting adequate air cleaning.